Analog-to-digital converters (ADCs) convert samples of an analog input signal into digital values corresponding to the samples. ADCs may be used in various electronic devices, such as finite impulse resolution (FIR) digital to analog converters (DACs) and many other types of electronic devices. A digital-to-analog converter (DAC) is a device for converting a digital signal to an analog signal.
A delta sigma (or sigma delta) modulation is a method for encoding analog signals into digital signals as found in an ADC. It can also be used to transfer high bit-count low frequency digital signals into lower bit-count higher frequency digital signals as part of a DAC.
A quantizer of a delta sigma modulator generates a digital representation of an input generated based on a sample of an analog signal via a conversion process. One or more DACs can convert the digital outputs to corresponding analog values, and the input to the quantizer is updated based on the analog values.
Continuous time delta sigma modulators generally suffer from excess loop delay (ELD). The ELD of a delta sigma modulator may correspond to a delay period associated with the one or more DACs, a delay period of the components that provide the input to the quantizer, and a delay period of the quantizer itself. The delay period of the quantizer may correspond to a period between when the quantizer is prompted to update the digital outputs and when the quantizer actually outputs the updated digital outputs.
A conventional method for compensating ELD of a delta sigma modulator includes providing an additional DAC and one or more delay buffers. However, this conventional method requires at least two DACs, which increases power consumption of the delta sigma modulator.
In addition, DACs tend to generate errors correlated to the digital input. These errors are typically the result of component mismatches, process and thermal gradients, and other non-linear error sources. These errors may create harmonic distortion that causes undesirable effects to the conversion process. Dynamic element matching (DEM) may be used to compensate for components mismatch and improve the average linearity of a DAC. For example, the one's in a given thermometer code are spread around in a random fashion by DEM so that the errors in the DAC currents are averaged together.
DEM can be used for both continuous time and discrete time delta sigma modulators to minimize DAC mismatch. But existing approaches for implementing DEM with a quantizer in a delta sigma modulator will cause a high timing constraint for the DEM and/or quantizer function. For example, both functions of the DEM and the quantizer have to finish within a same half clock cycle for a feedback compensation loop, which again needs higher power to speed up the operations of the DEM and/or the quantizer.
Thus, conventional techniques for ELD compensation with DEM implementation in a continuous time delta sigma modulator are not entirely satisfactory.